Nitric oxide (NO) is a mediator involved with diverse biological responses including nonspecific defense mechanisms, the regulation of vascular tone and neurotransmission. More recent findings suggest that NO may also regulate cell proliferation and secretory function. In studies addressing the potential role of NO in the pathogenesis of arthritis we have shown that human articular chondrocytes and cartilage tissue but not synoviocytes or inflammatory cells from synovial effusions can be induced to produce NO. In chondrocytes NO synthesis has the characteristics of inducible NO synthase (iNOS) and can be stimulated by proinflammatory cytokines. We cloned the cDNA encoding human chondrocytcs inducible nitric oxide synthase (iNOS). The sequence is identical to that of the human hepatocyte iNOS. We have demonstrated that NO can increase PGE2 production in chondrocytes, mediates the growth inhibitory effects of IL-I and that NO is the major inducer of apoptosis in chondrocytes. Using iNOS as a model, we have characterized signaling pathways that are utilized in the induction of iNOS and provided new insight into IL-I triggered intracellular events. Based on these preliminary findings the specific aims of this study are to (i) study regulation of NO synthesis and expression of the iNOS gene in chondrocyte and cartilage cultures; (ii) evaluate the effects of NO in the regulation of chondrocyte differentiation and secretory function and determine the role of NO in IL-I signal transduction; (iii) analyze iNOS expression in human arthritic cartilage and (iv) study the role of NO in cartilage degradation and chondrocyte apoptosis by using IRF-knock- out-mice which are deficient in iNOS expression. Information generated in these studies will provide new insight into the molecular and cell biology inducible NO synthase, the role of NO in the regulation of chondrocyte function and its contribution to the pathogenesis of cartilage destruction in rheumatoid arthritis and osteoarthritis. The results on the inducible nitric oxide synthase gene and the regulation of its expression have the potential to provide the basis for the development of novel therapeutic interventions against cartilage destruction.